Using preliminary work to choose
the values of control variables
Consider a simple experiment – like measuring the
temperature rise in a reaction between Copper
Sulphate and Magnesium powder. We will change
the mass of magnesium.
In this investigation, you will need to control some of the variables.
Write down one variable that will need to be controlled.
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Describe briefly how a preliminary investigation could help you to find a suitable
value for the control variable.
You should explain how the results of this preliminary investigation will help you
to decide on the best value for the control variable.
The rules:
Get it clear in your mind what the
following variables are:
Independent
Mass of
magnesium
Dependent
Temperature rise
Now identify the control variables:
• Volume of Copper Sulphate
• Concentration of Copper Sulphate
• Starting Temperature
Pick one that is easy to understand
Eg Volume of copper sulphate
To get the marks for this question:
Choose two values for the volume – a big one and a
small one would be good.
I will try a volume of 10cm3 of copper sulphate and a
volume of 100cm3 of copper sulphate
Say what you will do with these two volumes:
I will measure the starting temperature for both
volumes. I will add one spatula of magnesium powder to
each volume and measure the temperature after.
It is important to say what you are
going to measure
To get the final mark – say how you will decide which is
the best to use.
Whichever volume gives the bigger rise in
temperature will be the best one to choose